The invention relates to a method of braking a hydrostatic drive and to such a hydrostatic drive.
Hydrostatic drives are often used to drive the hydraulic systems for implement operation or the vehicle drive systems in construction site vehicles. In this case, a primary drive source, for example an internal combustion engine, is connected to at least one hydraulic pump. For driving the vehicle, a hydraulic motor is mostly connected to this hydraulic pump in the closed circuit. Because of the mass inertia of the vehicle, during braking of the vehicle the hydraulic motor, which is then driven by the vehicle wheels, acts as a pump and without reversing the direction of flow delivers pressure medium in the closed hydraulic circuit. The hydraulic pump, which is therefore loaded with the pressure medium at its suction side, accordingly acts as a hydraulic motor and generates an output torque, which counteracts the internal combustion engine.
In vehicles travelling at low speed, such as for example forklift trucks, the hydrostatic transmission of the vehicle drive system is often also used to brake the vehicle. For this purpose, it is known for the hydraulic pump to counteract the internal combustion engine. The brake power thereby achievable is limited by the available brake power of the internal combustion engine. From DE 198 92 039 A1 it is further known to connect a second, variable displacement hydraulic pump to the pump shaft. This second, variable displacement hydraulic pump is disposed in an open circuit and designed for delivery in one direction only. The pressure medium delivered by the second, variable displacement hydraulic pump may in the event of a braking operation be relieved via a pressure relief valve back into the tank volume. The volumetric flow towards the pressure relief valve is in this case limited by means of a valve, which with progressive actuation of a brake pedal supplies an increasing volumetric flow to the pressure relief valve.
The drawback of the described drive system is that, in addition to the hydraulic pump and the hydraulic motor and the respective drive circuits thereof, a further hydraulic pump is needed to generate a braking effect. Besides the adjustment of the hydraulic pump and the hydraulic motor of the hydrostatic transmission, it is therefore additionally necessary to adjust the delivery rate of the further hydraulic pump. What is more, the delivery flow of the additional hydraulic pump that is relieved via the pressure relief valve has to be regulated by means of an adjustable valve. The braking operation therefore involves not only the vehicle drive system but also the open circuit needed for the actuation of hydraulic systems for implement operation.